1. Field of the Invention
The present invention relates to a cylinder for an internal combustion engine and to a method of treating the inner surface of the cylinder. In particular, the present invention relates to an internal combustion engine made of an aluminum alloy and having a plated inner surface constituting a piston-sliding surface, and to a method of treating the inner surface of the aluminum alloy cylinder.
2. Description of the Related Art
As a typical example of the conventional cylinder for a small air-cooled two-stroke gasoline engine to be employed in a portable power working machine, there is known a cylinder as shown in FIG. 4 (which also illustrates one embodiment of the present invention as explained hereinafter). The cylinder 1 shown in FIG. 4 is formed of an aluminum alloy and constituted by an integral body consisting of a main body 2 provided with a pair of columnar protruded portions 2a disposed diametrally opposite to each other, a head portion 3 having a squish dome-shaped combustion chamber 4 formed therein, and a large number of cooling fins projecting from all over the outer wall of the integral body. Further, the head portion 3 is provided therein with an internal thread 18 for mounting an ignition plug.
The main body 2 is provided, on the inner surface 9 thereof with which the piston of the engine is slidably contacted (or the surface of cylinder bore), with a suction port 5 and also with an exhaust port 6, both of which are designed to be closed or opened by the movement of the piston (indirected in phantom lines at 15). The suction port 5 and the exhaust port 6 are disposed to face each other in an off-set manner so that they disagree in level from one another. Furthermore, the main body 2 is also provided with a pair of columnar expanded portions 2a, in each of which a scavenging duct 7 accompanied by an inner wall 7a having a predetermined thickness (hereinafter referred to as inner wall-attached scavenging duct) is formed, these scavenging duct 7 being displaced in the circumferential direction of the cylinder bore 9 from the suction port 5 and the exhaust port 6 by an angle of 90 degrees. The downstream end portion (upper end portion) of each scavenging duct 7 is constituted by a scavenging port 8, thereby providing a pair of scavenging ports 8 which are disposed opposite to each other and designed to be opened and closed by the piston 15. The scavenging ports 8 are inclined somewhat upwardly in the direction opposite to that of the exhaust port 6 of the cylinder bore 9.
The cylinder disclosed in FIG. 4 is a so-called Schnürle binary fluid scavenging type cylinder, wherein a pair of scavenging ports 8 are symmetrically formed with respect to the longitudinal section taken along the middle of the exhaust port 6. Additionally, there is also known a so-called quaternary fluid scavenging type cylinder where another pair of scavenging ports are additionally provided therewith (two pairs of scavenging ports in total). As for the type of the scavenging duct, there are known a scavenging duct provided with an inner wall 7a as shown in FIG. 4, and a scavenging passageway having no inner wall 7a (the cylinder bore surface 9 side is opened). There is also known a half-wall, attached scavenging duct which is featured in that it is provided at a lower portion thereof with an opening extending in the longitudinal direction of the scavenging duct while leaving a half-wall having a predetermined thickness at an upper portion thereof so as to allow an air-fuel mixture introduced into the scavenging port from the crank chamber via the scavenging duct to be contacted with a skirt portion of the piston.
The cylinder 1, which is designed to be employed in a two-stroke internal combustion engine and made of an aluminum alloy as described above, is generally subjected to plating treatment (for forming a plated layer 10) subsequent to the cast molding thereof, by means of a high-pressure die casting method for instance, so as to enhance the abrasion resistance of the cylinder bore surface 9 with which the piston 15 is slidably contacted.
As for the plating treatment of the cylinder bore surface 9 of the cylinder 1, nickel (Ni)-based plating or chromium (Cr)-based plating using DC power as a power source have been chiefly employed up to date. However, the plating using DC power is accompanied by a problem in that, when the plating using DC power source is applied to the cylinder bore surface 9, a protuberance 10a is generated in the plated layer 10 at the fringe portion of the opening of a port as illustrated in FIG. 3, where the opening of a port portion (herein, the suction port 5 is shown as a representative example) such as the suction port 5, the exhaust port 6 and the scavenging ports 8 each penetrating through the cylinder bore surface 9 is shown. In addition to that, the plating using DC power is also accompanied by a problem in that whisker-like protrusions 10b called “carbon trumpet” are generated (FIG. 3), thereby making the thickness of the plated layer non-uniform and also roughening the surface of the plated layer. If such protuberance 10a is permitted to exist in the plated layer 10, resulting in non-uniformity and roughness in the surface of the plated layer, the sliding properties of the piston would be deteriorated, making the cylinder unsuitable for practical use.
Therefore, it has been conventionally practiced to form the plated layer 10 in such a manner that, by taking the finishing allowance into consideration, the plated layer 10 is initially formed relatively thick, and then, the resultant plated layer 10 is subjected to grinding work such as honing work of the cylinder bore surface 9 or chamfering work of each of the port portions, thus resulting in increase in manufacturing cost of the cylinder for an internal combustion engine.